Hypothalamic neuroendocrine cells develop patterned electrical activity under certain physiological conditions, which leads to a pulsatility of hormone secretion into the blood. Glutamatergic and noradrenergic synaptic mechanisms appear to play an important interactive role in the generation of bursting electrical behavior in these cells. Preliminary evidence suggests that magnocellular neuroendocrine cells of the paraventricular (PVN) and supraoptic nuclei (SON) receive excitatory synaptic inputs from norepinephrine-sensitive glutamate interneurons located within the respective nuclei. This suggests that local glutamate neurons may relay excitatory signals from noradrenergic afferents to the magnocellular neuroendocrine cells, which provides a potential mechanism for the initiation and synchronization of bursting activity in these cells. This proposal will test the hypothesis that synchronous burst generation among hypothalamic neuroendocrine cells is the result of noradrenergic activation of local glutamatergic synaptic inputs. The specific aims are to determine whether 1) norepinephrine excites magnocellular neuroendocrine cells of the SON by activating intranuclear glutamate circuits; 2) magnocellular neurons of the SON and paraventricular nucleus (PVN) are synaptically coupled via internuclear glutamate circuits. Whole-cell patch-clamp recordings from magnocellular neuroendocrine cells will be performed in acute hypothalamic slices. Intra-and internuclear circuits will be activated using electrical and focal chemical stimulation techniques. Population responses will be studied with multiunit recordings and calcium imaging to detect synchronized synaptic inputs to magnocellular neurons from local glutamate circuits. Magnocellular neurons will be identified as oxytocin and vasopressin cells with intracellular dye injection and post-hoc immunohistochemical labeling. Determining the mechanisms responsible for the generation of specific bursting patterns among different neuroendocrine neuronal populations is one of the main objectives of the physiological study of neuroendocrine systems. Pulsatility, a hallmark of neuroendocrine systems, is caused by patterned electrical activity in the hormone-secreting cells. The development of therapeutic strategies for treating disrupted or abnormal hormonal cycles requires an understanding of the mechanisms responsible for the generation of patterned activity in these cells. This proposal should make a substantial contribution to understanding the mechanisms of burst generation in hypothalamic neuroendocrine cells.